Neurons may be particularly prone to DNA damage by reactive oxygen species due to their high metabolic activity and low levels of antioxidant defenses [1]. Repair of oxidative DNA damage is therefore essential for normal brain function. Very little is known about neuronal DNA repair and therefore it is an important field for investigation. An etiological link to DNA damage via oxidative stress has been implicated in the pathogenesis of Parkinson's disease (PD) [2, 3]. PD is a progressive neurodegenerative disorder that is pathologically characterized largely by the loss of dopaminergic neurons of the substantia nigra. The initial underlying mechanism(s) that triggers neurodegeneration in PD is unknown. Elevated levels of DNA damage were detected in the dopaminergic neurons of the substantia nigra in PD patients [4-6]. It is unclear whether DNA damage is responsible for neuronal loss or is an epiphenomenon of the disease in the surviving neurons. Expression of "GO" enzymes (OGG1, MUTY, and MTH1), proteins involved in the repair of oxidative DNA damage, were also found to be increased in the dopaminergic neurons in the substantia nigra of PD patients [7- 9]. However, the extent to which the GO system acts to prevent DNA damage and/or mutations in both the nuclear and mitochondrial genomes in neurons is presently unclear. The proposed experiments will test the hypothesis that DNA damage is an early event in dopaminergic cell loss in the substantia nigra and that the GO pathway is important in protecting against such oxidative DNA damage. If DNA damage is potentially an underlying mechanism of neuronal degeneration, and GO repair is important in preventing this damage, these represent novel targets for the development of treatments to slow the progression of PD. A combination of molecular, biochemical and cellular techniques using rotenone models of PD and human postmortem brain tissues will be utilized. This proposal has the following two specific aims: (1) Determine the temporal and spatial role of DNA damage in the progressive loss of dopaminergic neurons;and (2) Determine the role of the GO members (OGG1, MutY, Mth1) in the repair of rotenone-induced DNA damage in both in vitro and in vivo models of PD.